Illumination based on light emitting diodes (“LEDs”) has a variety of advantages over conventional incandescent and fluorescent illumination, for both commercial and residential settings. LED lighting is inherently much more efficient than incandescent illumination since virtually all of the optical power emitted by LED emitters is distributed in the visible spectrum. While fluorescent bulbs (including compact fluorescent or “CFL” bulbs) are comparatively more efficient than incandescent bulbs, the spectrum of light emitted by fluorescent sources is still viewed as unpleasant as it fails to mimic the spectrum of sunlight. Additionally, fluorescent light sources (like incandescent sources) are fragile, requiring glass envelopes to maintain a vacuum and/or to contain the discharge gas. In the case of CFL bulbs, these bulbs contain trace amounts of mercury, which can be released when the bulbs break. Additionally, fluorescent lights take more time to fully illuminate.
LED lighting has the potential to overcome all of these disadvantages with conventional lighting sources. As is set forth above, LEDs are inherently efficient. Additionally, “white” LEDs, which are constructed by providing a short wavelength LED source which excites a phosphor emitter, can be “tuned” and then filtered to achieve a variety of pleasing color temperatures. LEDs are mechanically sturdy. Additionally LEDs can be driven in DC mode, eliminating flicker.
Given the advantages of LED illumination, LED illumination fixtures are increasingly being used in commercial and/or industrial environments. As a result of various statutory and practical safety requirements, lighting fixtures used in commercial and industrial settings generally must be equipped with a backup system to provide a minimum level of illumination in the event of a power failure. Conventional battery backup systems for driving fluorescent illumination fixtures are unsuitable for use with LED fixtures, which have vastly different current, voltage and drive characteristics.
Conventionally available LED lighting fixtures are provided as series combinations of LEDs, typically driven in DC. LEDs have I-V characteristics that are similar to other semiconductor diodes, which is to say, that current varies exponentially as a function of forward voltage. The “white” LEDs used in commercial lighting applications typically drop 3.3 to 4.5V at their ideal operating forward current. Commercially available DC LED lighting fixtures are available in 12 VDC, 20 VDC and 30 VDC, representing different numbers of series connected LEDs. Conventional emergency backup drivers for DC LED lighting fixtures are provided as constant current devices, for example, by varying load power. However, because DC LED lighting fixtures require a variety of different operating voltages, a variety of conventional emergency backup drivers are required to supply the necessary current at the voltage required by the specific lighting fixture to be driven. It would be desirable to provide an emergency backup LED driver capable of supplying backup power to LED lighting fixtures regardless of forward voltage.